1. Field of The Invention
The present invention relates to a laser beam transmission through flexible optical waveguides, more particularly to a method and apparatus for inputting a high power laser beam into optical fiber end faces or vice versa with high efficiency (i.e., low reflection loss) and reducing optical damage to the fiber ends.
2. Description of The Prior Art
In conventional optical fiber beam delivery system, a laser beam is directly focused onto the end face of an optical fiber which is optically polished.
However, due to refractive index mismatch of the media at the interface (i.e., air and waveguide material), Fresnel reflection, which is given for normal incident light beam by R=(n-1).sup.2 /(n+1).sup.2 where n is the relative index of refraction of the interface, brings about coupling loss of the laser power. In addition, micro-irregularity on the fiber end faces resulting from polishing offers an additional channel for coupling loss due to scattering.
Moreover, the laser beam is focused onto the input end of the fiber resulting in development of extremely high power density at the interface region. As a result, optical damage to the fiber end surfaces poses a limit to the input power level for safe transmission. In particular, micro-irregularity (or roughness) on an optical surface can severly reduce the optical damage threshold of the surface. Therefore, it was hard to input the high power laser beam into the fiber end face or vice versa.
There is another problem at both input and output ends of the fiber as a high power laser beam is transmitted through it. Continual burning of cladding of the fiber at both input and output ends due to overheating of the cladding has been reported. This problem may be attributed to mode-mismatch between the transverse modes of the focused laser beam and the guided (core) modes of the fiber. Namely, if the transverse mode (i.e. spatial distribution) of a focussed light beam incident on the fiber end face is not completely matched with the spatial distribution of the guided mode 1 shown in FIG. 1, the focussed light beam may excite in the fiber not only the guided mode 1 but also the whispering-gallery modes 2 shown in FIG. 2, wherein the fiber 3 comprises the core 4 and the cladding 5. Therefore a light beam, after being input into a fiber, propagates through the fiber partially in the guided modes and partially in the whispering-gallery modes, so that the guided modes will be emitted outwards from the other end of the fiber and the whispering gallery modes will be trapped in the cladding layer of the fiber. Consequently a significant portion of the laser power is trapped in the cladding layer, resulting in excessive heat deposition in this layer due to optical absorption.